Fuel 81 (2002) 1499±1508
CCSEM analysis of ash from combustion of coal added with limestone q Lian Zhang, Atsushi Sato, Yoshihiko Ninomiya* Department of Applied Chemistry, College of Engineering, Chubu University, Matsumoto-cho, 1200 Kasugai, Aichi, Japan Received 10 May 2001; revised 24 October 2001; accepted 13 November 2001; available online 10 April 2002
Abstract Combustion of three Chinese coals, mixed with limestone physically, was carried out in drop tube furnace. The drop tube furnace consisted of two parts, the top side has a length of about 1.0 m and kept at 1573 K in all the runs, while the bottom-side has a length of 0.5 m and kept at 1173 K. SO2 removal ef®ciency of about 80 and 73% were obtained in the combustion of Yanzhou with high and low sulfur, respectively. In contrast, for Datong coal, the De-S ef®ciency was only about 50% at the molar Ca/S ratio of 2.0; increasing Ca/S ratio to 3.0 had little effect on De-S ef®ciency. The combustion ashes were analyzed by several techniques including XRD, SEM-EDX and CCSEM (computercontrolled SEM). A novel calcium-based phase de®nition, based on CCSEM data was developed to investigate the modes of occurrence of added limestone in the ashes. Additionally, the mixture of limestone with kaolinite was injected into the furnace to study their transformation behavior under simulated coal combustion conditions. The governing mechanisms for limestone capturing sulfur and its reaction with the inherited minerals were correspondingly revealed. It was found that under the given coal combustion conditions, the calcium distribution in the ash varied with coal type and residence time. Brie¯y, more calcium was used for desulfurization or ®xed into mineral; as time progressed, the inherited aluminosilicate, small sized excluded particles in the coal matrix, facilitated its reaction with limestone; it also reacted quickly compared to sulfation of limestone in coal combustion. This in turn hampered the ef®cient utilization of limestone in coal combustion. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Coal combustion; Desulfurization; CCSEM; Calcium utilization ef®ciency
1. Introduction With the requirement of energy getting increased rapidly, switching to cheaper coals, normally, those with high sulfur and high ash contents, has led to signi®cant increase of SO2 emission in coal combustion . This is more serious in some non-OECD countries, where a number of small boilers are burning the indigenous low-grade coals, and ¯ue gas desulfurization (FGD) were not built in the old plants [2,3]. Even in the developed countries, since the trade for the high sulfur coal is expected to increase, use of the lowgrade coal in combustion will signi®cantly enhance the load of secondary pollutant control in FGD. For example, to meet the current European Community Standard of 400 mg/m 3, changing to a coal with 5.0% sulfur content will require approximately 90% SO2 removal, while changing to a coal containing 1.0% Sulfur requires about 50% SO2 removal . Considering the load of FGD and its process cost, it is, therefore, necessary to decrease the SO2 concentration in the outlet ¯ue gas generated from coal combustion. * Corresponding author. Tel.: 181-568-51-9178; fax: 181-568-51-3833. E-mail address: [email protected]
(Y. Ninomiya). q Published ®rst on the web via Fuel®rst.comÐhttp://www.fuel®rst.com
Because of its low cost and simplicity, sorbent injection process was the primary candidate for desulfurization . Calcium-based sorbent is most widely used and the preferred sorbent is limestone due to its low cost. Since 1960s, the behavior of limestone in coal combustion furnace has been given much attention . Under oxidizing conditions, limestone, at ®rst, calcines to porous lime, which then quickly captures SO2 in the ¯ue gas and forms gypsum. Since the gypsum molar volume is greater than that of lime, its existence on the surface of lime blocks the porosity of lime ; additionally, lime undergoes sintering under high temperatures , these lead to a low calcium utilization ef®ciency. Other parameters, such as Ca/S ratio, sorbent size and operating conditions also play signi®cant roles in the behavior of limestone . In order to improve the Sulfur removal ef®ciency, researches continue to ®nd better sorbents. The use of calcium organic salts and mixture of ¯y ash and limestone have been suggested as better sorbents to be used in FGD [7,9,10]. Besides the above parameters, the inorganic minerals in coal may also affect the fate of added limestone in coal combustion . This was, however, given little attention up till now. Since the temperature is above 1473 K in coal combustion, a reasonable part of calcium reacts with inherited aluminosilicates or other minerals. Moreover,
0016-2361/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0016-236 1(02)00065-0
L. Zhang et al. / Fuel 81 (2002) 1499±1508
Table 1 Proximate and ultimate analysis of three chinese coals YZHS Proximate (wt% on dried base) Ash 9.9 Volatile matter 39.9 Fixed carbon 50.2 Ultimate (wt%, daf) C 76.4 H 5.6 N 1.4 S 5.7 O (by difference) 10.9
13.9 26.8 59.3
10.0 35.5 54.5
78.6 4.9 0.8 1.7 14.0
79.7 5.5 1.5 0.6 12.7
considering the reports that calcium aluminosilicate are more active than pure limestone to absorb SO2 [8,10,12], it is possible to have large amounts of SO2 ®xed in the residues in coal combustion. To prove the above concept and investigate the effect of inherent coal minerals on limestone utilization, combustion of three coals, having different sulfur and mineral contents, were conducted in drop tube furnace (DTF). Limestone was used as sorbent which was mixed with coal physically. The temperature of DTF on the top was controlled above the coal ignition temperature, while temperature on the bottom was controlled in the limestone desulfurization temperature window. This paper aims to elucidate the fate of added limestone in coal combustion. The mechanisms governing calcium transformation were discussed here.
2. Experimental 2.1. Property of materials Three coals from China, Yanzhou with high sulfur (YZHS), Datong (DT) and Yanzhou with low sulfur
(YZLS) were pulverized to ,250 mm before use. Proximate and ultimate analyses of these are summarized in Table 1. Apparently, the sulfur and mineral contents in these three coals vary signi®cantly. A kind of Chinese limestone was used as sorbent and mixed with raw coal physically. It was screened to less than 63 mm before use. For YZHS coal, the molar ratios of added calcium to sulfur were 1.0 and 2.0, for DT 2.0 and 3.0, respectively, while for YZLS, it was kept at 2.2. To investigate the reaction between inherited minerals with added limestone, a Japanese kaolinite was used as a model compound and it was screened to 63 mm before use. 2.2. Combustion conditions Coal combustion was carried out in DTF and the experimental apparatus is shown in Fig. 1. Brie¯y, the reaction tube consisted of two parts, the top-side is about 1.0 m in length while bottom-side has a length of about 0.5 m in all the runs, the top-side of furnace was heated to 1573 K, higher than the coal ignition temperature, while the bottom-side was controlled at 1173 K for sulfation of limestone. This condition is relatively similar to the temperature gradient in pulverized coal boilers. The raw material, at a feeding rate of about 0.1 g/min, was entrained by air into the furnace; the air ¯ow rate was selected as 2.0±10.0 l/min; a mixture of N2 and O2 at a volume ratio of 4:1 was used as the combustion atmosphere. Residence time was controlled by the total gas ¯ow rate; the exit gas was trapped into H2O2 (1/ 99) solution and analyzed by Ion chromatography for outlet SO2 concentration. Fly ash was collected by vacuum suction system. 2.3. Sample analyses Fly ash taken from coal combustion was analyzed by XRD, SEM-EDX and CCSEM. The Philips PW 1825/00 was used for X-ray diffraction. The XRD patterns were collected at a voltage of 40 kV and a current of 35 mA. SEM-EDX and CCSEM were carried out by the JEOL scanning electron microscope model JEM-5600 with CDU-LEAP. For CCSEM analyses, all the materials were mounted in epoxy resin, cross-sectioned and polished, and then were coated with carbon layer to eliminate the electrostatic effects. Three magni®cations, 150 for the size range of 22.0±211.0 mm, 250 for the size range of 4.6±22.0 mm and 800 for the size range of 0.5±4.6 mm were used to obtain the backscattered image of samples. 3. Results and discussion 3.1. Mineralogical properties in raw coals
Fig. 1. Structure of DTF used in coal combustion.
The mineral composition, size and their association with organic materials in the raw coals were analyzed by CCSEM. As summarized in Fig. 2, YZHS is rich in an unknown compound having non-stoichiometrical Ca, Al
L. Zhang et al. / Fuel 81 (2002) 1499±1508
Fig. 2. Main mineralogical composition in three Chinese coals.
and Si elemental contents, pyrite and gypsum, while in the other two coals, aluminosilicate dominates the mineralogy; DT is rich in quartz and kaolinite, and YZLS is mainly rich in kaolinite. The particle size distribution of minerals and the in/excluded ratio of main minerals are shown in Fig. 3 and Table 2, respectively. Clearly, the size of minerals in YZLS is the biggest in the three coals and most of them were included in the organic material of raw coal. On the other hand, the mineral in DT has the smallest size and more than half was isolated from organic material of raw coal. The size of mineral in YZHS falls between that of DT and YZLS, and the included to excluded weight ratio is about 50:50. 3.2. SO2 removal ef®ciency in coal combustion SO2 emission in the combustion of raw coals and that added with limestone, analyzed from gas phase, was shown in Fig. 4. The SO2 removal ef®ciency is de®ned as
Fig. 3. Particle size distribution of minerals in three Chinese coals.
100 minus the percentage of SO2 concentration generated from combustion of coal added with limestone to that generated from combustion of raw coal. It is obvious that the removal ability of limestone varied with coal type and residence time. Generally, as time progressed, more SO2 was captured by limestone. For YZHS and YZLS, with the addition of limestone, a long residence time of 1.8 s made SO2 emission to decrease sharply compared to that at short times. Correspondingly, the SO2 removal ef®ciency of 80% and 73% was achieved for YZHS and YZLS, respectively. On the other hand, addition of limestone only led to about 50% of the total sulfur captured during the combustion of DT. Even as Ca/S molar ratio increased to 3.0, there was a little enhancement in SO2 removal ef®ciency (shown in Table 3). This difference obviously resulted from the coal characteristics. After added with coal into furnace, limestone may undergo complex chemical and physical transformations; these reactions to a signi®cant extent depend on the chemical forms of sulfur in raw coal and the characteristics of minerals as well. It is, therefore, necessary to investigate the mode of occurrence of calcium after coal combustion. 3.3. Mode of occurrence of calcium in the ash Quantitative XRD analysis of typical ashes, gotten from the combustion of raw DT and that added with limestone at 1.8 s is shown in Table 4. The results showed the prevalence of quartz, mullite and hematite in both the ashes; addition of limestone led to lime, anhydrite and anorthite formed in the ash; existence of lime was due to the decomposition of limestone in coal combustion, while anhydrite was formed by sulfation of CaO and anorthite gotten from interaction of limestone with inherited al-silicate in raw coal. Apparently, added limestone underwent several reactions in coal combustion; it decomposed to crystal lime, captured sulfur and a part of it was ®xed into calcium aluminosilicate. Since XRD can only detect out the crystalline phases in ashes, and therefore, how to detect the amorphous glasses, and how to exactly understand the quantitative distribution of calcium cannot be gotten here, but via CCSEM analysis will be shown below. SEM photography of calcium-based compounds in ash from combustion of YZHS with limestone at 1.8 s was shown in Fig. 5. Clearly, limestone used here was fragmented and decomposed into porous lime in coal combustion; and its surface was used for capturing sulfur. Fine CaSO4 particles, with a diameter of about 1.0 mm were formed as a round shape and agglomerated together. Obviously, CaSO4 was in the molten phase or at least has a sticky surface under given high temperatures; it may adhere to the surface of other particles or bind with each other. This was in accordance with the fact that at temperatures between <1170 and 1300 K sulfate liquid phases play a role in the liquid phase formation . Similarly, calcium aluminosilicate showed as round particles in the ash; though quartz and kaolinite were kept unaltered, if subjected to high temperature gases,
L. Zhang et al. / Fuel 81 (2002) 1499±1508
Table 2 In/excluded ratio of main minerals in three Chinese coals YZHS
Quartz Kaolinite Pyrite Unknown
18.5 18.6 13.5 45.9
81.5 81.4 86.5 54.1
24.9 21.9 97.8 43.1
75.1 78.1 2.2 56.9
94.0 87.0 ± 57.1
6.0 13.0 ± 42.9
the presence of alkali made the formation of liquidous calcium aluminosilicate possible [13,14]. In addition, calcium aluminosilicate containing sulfur, de®ned as gypsum/aluminosilicate, was also formed as molten phase; it condensed into round particle. The formation of gypsum/ aluminosilicate might be due to (1) calcium sulfate binds
with melt al-silicate; (2) calcium aluminosilicate has the ability of capturing sulfur. The possibility of above reaction routes will be investigated brie¯y later. The SEM photography also showed that the eutectic compound, Ca±Fe±S in molten phase was formed. The quantitative calcium distribution was analyzed by CCSEM. As described widely before , CCSEM scans thousands of particles in order to produce statistical information on certain elements. Since only calcium-containing compounds and the inherited al-silicate were of concern here, a novel calcium-based compounds de®nition category was developed, which was shown in Table 5. Brie¯y, calcium-containing compounds in ash were categorized into three kinds: the unreacted includes lime and Ca-rich, in Table 5, sulfation products include gypsum, gypsum/ aluminosilicate and Ca±Fe±S eutectic compound, while calcium aluminosilicate was assigned as mineralized. The particle size distribution of calcium-based compounds in the ashes of YZHS with limestone at different residence time is shown in Fig. 6. For comparison, the particle size distribution of initial limestone and that of inherited aluminosilicate in raw coals are shown there too. The dotted line in it is the particle size distribution of excluded kaolinite in raw YZHS. As shown in Fig. 6, at 0.4 s, the amount of the unreacted limestone was kept as great. Compared to the initial limestone, the particles of limestone less than 10 mm disappeared, the existence of calcium aluminosilicate implied that a part of it was captured by inherited alsilicate. The gypsum and gypsum/aluminosilicate shown at 0.4 s were those in the raw coal. As residence time increased to 1.1 s, more gypsum and gypsum/aluminosilicate were formed in the ash. There was little change of the amount of calcium aluminosilicate compared to that at 0.4 s. Additionally, the gypsum formed here has S/Ca molar ratio less than 0.8. Gypsum/aluminosilicate, however, agglomerated Table 3 De-S ef®ciency in coal combustion at 1.8 s
YZHS DT Fig. 4. SO2 emission in combustion of three coals.
1.0 2.0 1.9 3.0 2.2
82.0 84.0 55.8 60.5 73.0
L. Zhang et al. / Fuel 81 (2002) 1499±1508 Table 4 XRD analysis of crystalline phases in ashes of DT coal at 1.8 s
Mullite Quartz Hematite Lime Anorthite Anhydrite
20.3 35.9 15.7 ± 6.9 2.3
13.7 18.1 16.3 10.8 11.6 8.3
into large particles. As residence time further increased to 1.8 s, the amount of gypsum decreased while more than 50% of added limestone was ®xed into gypsum/aluminosilicate, which has the mode size of 50.0 mm. Compared to the prevalence of gypsum at 1.1 s, although the formation route of gypsum/aluminosilicate was still unknown, it was possible that much calcium was captured by aluminosilicate at longer residence time, the formed calcium aluminosilicate, rich in calcium, absorbed SO2 at the bottom of the reaction tube. Being a liquid, gypsum/al-silicate agglomerated into larger particles. The calcium distribution in ashes of DT added with limestone is presented in Fig. 7. In the mixture of raw DT with limestone, kaolinite and quartz are smaller in size than that of added limestone. Additionally, most of them exist as excluded in DT coal. The particle size distribution of calcium-based compounds at 0.4 s indicated that even at such short time, limestone was captured by the inherited ®ne al-silicate to form calcium aluminosilicate. The calcium aluminosilicate had bimodal size distribution, the particles having the mode size of 10.0 mm was mainly of single round
particles, while the bigger one formed by agglomeration. Compared to that in raw coals, the mode size of kaolinite at 0.4 s shifted to 100.0 mm; this substantiated the fact that ®ne aluminosilicate in raw coal facilitate its interaction with added limestone, as indicated by the previous work . At 1.8 s, most calcium was still kept as calcium aluminosilicate while there was little change in sulfation products. Compared to the above two coals, YZLS is rich in kaolinite, which shows bimodal size distribution and the ®ne ones exist as included particles (shown in Fig. 8). As combustion proceeded, the included kaolinite underwent coalescence to larger particles. Meanwhile, added limestone mainly underwent decomposition and sulfation. There seemed little interaction between limestone and the inherited kaolinite. The calcium utilization ef®ciency, calculated using above CCSEM data, was shown in Table 6. What was mentioned here was the mass balance in DTF. Since the calcium or sulfur containing compounds have low melting point and were formed as liquid in the tube, they easily adhere to the furnace inside. SEM photography indicated that they were rich in calcium and sulfur, and had an average size of about 1.0 mm. Calculation results showed that about 20% calcium and sulfur were lost after reaction, thus, all the above results in the semi-quantitative order. This, however, can still give us a deep understand on sorbent transformation in coal combustion. In conclusion, combined with the particle size distribution of calcium-based compounds in ashes shown in Figs. 6±8, Table 6 implied that calcium utilization ef®ciency varied with coal type and residence time. Generally, most of the limestone was kept unreacted for 0.4 s, while
Fig. 5. SEM photography of calcium-based compounds in ashes.
L. Zhang et al. / Fuel 81 (2002) 1499±1508
Table 5 CCSEM Ca-based compounds de®nition category (Gypsum-1 refers to gypsum having molar S/Ca $ 0.8, gypsum-2 refers to gypsum having molar S/ Ca , 0.8) No
Density (g/cm 3)
SEM-EDX criterion (wt%)
1 2 3 4 5 6 7 8 9 10 11
Lime Ca-rich Gypsum-1 Gypsum-2 Gyp-1/aluminosilicate Gyp-2/aluminoslicate Ca-aluminosilicate Ca Silicates Ca alumina Ca Fe±S Ca±Fe/aluminosilicate
2.8 2.6 2.5 2.7 2.6 2.6 2.65 3.09 2.8 2.7 2.7
Ca $ 80, Si , 10, Al , 5, S , 5, Fe , 5 65 # Ca , 80, S , 5, Fe , 5 Ca $ 50, S/Ca $ 0.64, Fe , 5 Ca $ 50, 0.1 , S/Ca , 0.64, Fe , 5 10 , Ca , 50, S/Ca $ 0.64, Fe , 5 10 , Ca , 50, 0.1 , S/Ca , 0.64, Fe , 5 Si . 10, Al . 5, 5 , Ca , 65, Fe , 5, S , 5 Si . 10, Al , 5, 5 , Ca , 65, Fe , 5, S , 5 Si , 10, Al . 5, 5 , Ca , 65, Fe , 5, S , 5 5 , Ca , 65, Fe . 5, S . 5 5 , Ca , 65, Fe . 5, S , 5
These implied that limestone and inherited kaolinite behaved in parallel in YZLS combustion. The receding of coal particle surface under high temperatures brought the included mineral particles together, and therefore, kaolinite underwent coalescence into larger particles. Meanwhile, added limestone underwent decomposition and reacted with SO2 released from coal particles. In contrast, in DT coal combustion, more than half of the added limestone was captured by the inherited aluminosilicate, and therefore, a lower sulfur removal ef®ciency of about 50% was achieved even at the Ca/S molar ratios of 2.2 and 3.0. Since most of aluminosilicate in DT exist as excluded particles, as combustion started, the added limestone was captured by aluminosilicate soon and ®xed into calcium aluminosilicate. In addition, the ®ne distribution of
increasing residence time to 1.1 s or longer made more calcium react with SO2 or inorganic minerals in raw coal. For YZHS coal, a high De-S was obtained even at Ca/S molar ratio of 1.0; this was partially due to the fact that the added limestone was in large amounts than that of inherited minerals. Additionally, the high SO2 concentration released in the YZHS combustion allowed for the highest calcium utilization ef®ciency . Similarly, a high sulfur removal ef®ciency was also achieved in the combustion of YZLS coal. Since the sulfur content in YZLS is as low as 0.57%, the weight ratio of added limestone to inherited kaolinite was very low; furthermore, most kaolinite existed as included and they underwent coalescence in combustion; there were few reactions between added limestone and kaolinite in coal combustion.
Fig. 6. Particle size distribution of calcium-based compounds and that of aluminosilicates in ashes of YZHS.
L. Zhang et al. / Fuel 81 (2002) 1499±1508
to carbonaceous materials in raw coal have signi®cant effect on the utilization ef®ciency of added limestone. The interaction between aluminosilicate and added limestone is important for understanding the transformation of limestone in the combustion system. In the following section, limestone and model kaolinite were mixed together and injected into DTF. Their behaviors under the simulated coal combustion conditions were then elucidated and the possible transformation of limestone was raised out. 3.4. Behaviors of model compounds under simulated coal combustion and transformation mechanism of added limestone in combustion system
Fig. 7. Particle size distribution of calcium-based compounds and that of aluminosilicates in ashes of DT.
kaolinite in DT facilitated its reaction with limestone. The lower calcium utilization ef®ciency implied that the calcium aluminosilicate formed in DT coal combustion could not capture SO2. Therefore, the particle size distribution of inherited minerals, especially of aluminosilicate and their association
Behaviors of limestone and model kaolinite were studied under the following conditions: the entraining air rate was selected as 2.0 l/min, while the mixture of N2 1 O2 (4:1) was used as a simulated combustion atmosphere. For De-S experiments, the mixture of N2 with SO2 at an SO2 concentration of 1% was used instead of pure N2. The temperature at the top of the furnace was ®xed as 1573 K while that at the bottom was ®xed as 1173 K in all the runs. In the mixture of limestone with kaolinite, the weight ratio of limestone to kaolinite was kept as 1.07. The De-S result of pure limestone and that of mixture of limestone with kaolinite were shown in Table 7. Ca/S molar ratio was kept as 1.7 in the runs. A De-S ef®ciency of 75.92% was obtained using pure limestone; this was similar to the De-S ef®ciency in the combustion of YZHS and YZLS, while mixing limestone with kaolinite apparently
Fig. 8. Particle size distribution of calcium-based compounds and that of aluminosilicates in ashes of YZLS.
L. Zhang et al. / Fuel 81 (2002) 1499±1508
Table 6 Calcium utilization ef®ciency in coal combustion
Unreacted (wt%) De-S (wt%) Mineralized (wt%)
YZHS (0.4 s)
YZHS (1.1 s)
YZHS (1.8 s)
DT (1.8 s)
YZLS (1.8 s)
60.0 12.5 27.5
8.0 81.0 11.0
9.0 79.0 12.0
20.0 26.5 53.5
61.0 37 2.0
decreased the De-S ef®ciency, which was 62.46% and similar to that in DT combustion. Behaviors of limestone and kaolinite under simulated coal combustions, expressed in terms of particle size distribution, were shown in Fig. 9. The lines marked raw denote the particle size distribution of initial limestone or kaolinite, without SO2, they denote to the particles size distribution of limestone and kaolinite under the combustion atmosphere of N2 1 O2 (4:1), while with SO2, they denote the particle size distribution of limestone and Kaolinite under the combustion atmosphere of N2 1 SO2 (1%) 1 O2. Clearly, the small sized limestone and kaolinite facilitate their transformation under combustion conditions. Limestone with particles less than 22.0 mm reacted with kaolinite and SO2; similarly, the kaolinite, less than 22.0 mm was prone to react with limestone. Furthermore, with the addition of SO2 in the combustion atmosphere, there was a large amount of coarse kaolinite (.22.0 mm) and a few ®ne ones (,22.0 mm) kept unreacted compared to that without addition of SO2. This, to a certain extent indicated that the sulfation of limestone takes places more quickly than the reaction between limestone and kaolinite. This phenomenon, however, was not consistent with what was observed in coal combustion. This was due to the difference in the chemical form of sulfur-containing compounds in combustion systems. SO2 was directly injected into DTF under simulated combustion conditions, while in coal combustion, SO2 was released from the decomposition of carbonaceous materials or the inorganic sulfur-containing compounds. The latter one was time-consuming . The particle size distribution of sulfation products and calcium aluminosilicates under simulated combustion conditions was shown in Fig. 10. Being consistent with the transformation of limestone, gypsum was produced with a mode size of about 5.0 mm for gypsum-1, which refers to Ca±S compounds having molar S/Ca $ 0.8, while gypsum-2, having molar S/Ca , 0.8, was produced to have a larger mode size at about 10.0±22.0 mm. Considering the molar volume ratio of CaSO4 to CaCO3, the particle size of initial limestone at about 3.5 mm is bene®cial
to be fully used. This is in accordance with what the others found . In addition, gypsum/aluminosilicate having bimodal size distribution was formed. Considering the complex reactions between calcium and aluminosilicate, calcium aluminosilicates were categorized into three groups: CaAlSi-1 is calcium aluminosilicate having molar Ca/Si $ 1.0, and CaAlSi-2 has molar Ca/Si in the range of 0.5±1.0, while CaAlSi-3 has molar Ca/Si , 0.5.
Table 7 De-S ef®ciency of model compounds under simulated coal combustion conditions
Blank Limestone Limestone 1 kaolinite
793.4 191.1 298.8
0 75.9 62.5
Fig. 9. Particle size distribution of limestone and that of kaolinite under simulated coal combustion conditions.
L. Zhang et al. / Fuel 81 (2002) 1499±1508
Fig. 10. Particle size distribution of products formed under simulated coal combustion conditions.
Table 8 Quantitative Ca distribution in ashes of model compounds under simulated coal combustion conditions (CaAlSi-1: 1.0 , Ca/Si , 10.0; CaAlSi-2: 0.5 , Ca/Si , 1.0; CaAlSi-3: 0.2 , Ca/Si , 0.5; Gypsum-1: S/Ca (mol/ mol) . 0.8; Gypsum-2: 0.1 , S/Ca (mol/mol) , 0.8)
Lime CaAlSi-1 CaAlSi-2 CaAlSi-3 Gypsum-1 Gypsum-2 Gyp-1/AlSi Gyp-2/AlSi
42.6 36.7 13.4 7.3 0 0 0 0
11.1 11.4 6.9 6.6 14.7 22.8 0.4 26.1
231.5 225.3 26.5 20.7 14.7 22.8 0.4 26.1
It was clear from Fig. 10 that the particle size of calcium aluminosilicate decreased with increasing molar Ca/Si in it. Adding SO2 in the combustion atmosphere made the amount of ®ne particles of CaAlSi-1, at the mode size of 4.6± 10.0 mm, to decreased signi®cantly, while there was a small change in the other two kinds of calcium aluminosilicates having molar Ca/Si less than 1.0. This was presented quantitatively in Table 8, in the third column of which was the difference in the amount of calcium-based compounds with the addition of SO2 in the combustion atmosphere to that without the addition of SO2 was shown. The similarity of decreased CaAlSI-1 amount to that of increased gypsum/ aluminosilicate further substantiated that calcium aluminosilicate, rich in calcium, has the capability to remove SO2. The formed gypsum/aluminosilicate has a bimodal size distribution which is shown in Fig. 10, the smaller one was attributed to the single particle while the larger one was caused by the agglomeration. The above results can explain the prevalence of gypsum/ aluminosilicates in the combustion ash of YZHS at 1.8 s, while calcium aluminosilicates dominated in the combustion ashes of DT coal. Combining the above data with that of the combustion of the three coals, it was possible to know about the transformation of added limestone and that of inherited aluminosilicate under given combustion conditions, which was described in Fig. 11. The reactions include (1) with the receding of the carbon surface, coalescence and shedding of individuals including Al-silicates takes place; this phenomena was con®rmed in the combustion of YZLS, which was also summarized by Benson et al.; (2) Limestone decomposes to porous lime, which then captures the released SO2 to form gypsum, while some part was kept unreacted; (3) The excluded al-silicate undergoes fusion to form melt phase, part of which then captures calcium to form calcium aluminosilicate, while some part binds with gypsum to form gypsum/aluminosilicate. The formed calcium aluminosilicates, having molar Ca/Si . 1.0, have the capability to capture SO2 too. The calcium utilization ef®ciency depends on the SO2 release rate, the particle size distribution of inherited aluminosilicate and their association to carbonaceous materials in raw coal. The molar ratio of calcium to al-silicate also plays a signi®cant role. 4. Conclusions Combustion of three Chinese coals, added with limestone, was conducted in DTF. The top of the furnace was controlled at 1300 8C while the bottom-side was at 900 8C. The results showed that at 1.8 s, a De-S ef®ciency of about 80% was achieved for YZHS coal added with limestone at molar Ca/S of 1.0 and 2.0, 73% for YZLS added with limestone at molar Ca/S of 2.2, while only 55% was achieved for DT added with limestone at molar Ca/S of 2.0. Increasing molar Ca/S to 3.0 had little effect on De-S ef®ciency. These
L. Zhang et al. / Fuel 81 (2002) 1499±1508
: added limestone
Fig. 11. Limestone transformation mechanisms in coal combustion.
differences were caused by the different calcium distributions in the combustion ashes of three coals. For YZHS, most of the calcium was used to capture Sulfur to form gypsum/aluminosilicate; for YZLS, most of the inherited aluminosilicate were included and underwent coalescence in coal combustion. Meanwhile, the added limestone underwent decomposition and sulfation in parallel. There was little reaction between them. In contrast, for DT coal, more than half of the added calcium was ®xed into calcium aluminosilicate and this was due to the fact that the aluminosilicate in DT coal was rich in ®ne particles and most of them existed as ®ne particles. The behaviors of model compounds, limestone and model kaolinite, were studied under the simulated coal combustion conditions. The results showed that the ®ne particles of limestone and kaolinite, less than 20.0 mm, were prone to react, while the coarse particles of both of them were kept unreacted. The particle size of limestone of about 3.5 mm was bene®cial to be used fully. Besides limestone, the calcium aluminosilicate having molar Ca/Si $ 1.0 also had the capability to capture SO2 in coal combustion; while calcium aluminosilicate having molar Ca/Si , 1.0 had no such capability. This signi®cantly hampered the calcium utilization ef®ciency. Furthermore, the SO2 release rate in coal combustion also played signi®cant role on the De-S ef®ciency. Acknowledgements The authors would like to thank the Grant-in-Aid for
Scienti®c research on Priority Areas (B), the Ministry of Education, Science, Sports and Culture of Japan for the ®nancial support. References  Rousaki K, Couch G. Advanced clean coal technologies and low value coals. IEA Coal Res 2000;November.  Soud HN. Developments in FGD. IEA Coal Res 2000;March.  Chen BH, et al. Environ Technol 1992;13(4):301.  Carpenter AM. Switching to cheaper coals for power generation. IEA Coal Res 1998;April.  Lin S-Y, Shoji O, et al. J Chem Engng Jpn 1996;75(8):718±31.  Wickert K. VGB 1963;83:74.  Juan A, Vanessa F, et al. Fuel 1997;76(3):257±65.  Fukasawa K. Low cost, retro®t FGD systems. IEA Coal Res 1997;September.  William N, Jonathan A, et al. Ind Engng Res 1999;38:2954±62.  Soud HN. Dev FGD IEA coal research, 2000;March.  Nilsson C, Lee BC. PFBC residues-characteristics, disposal and utilization. IEA Coal Res 1994;August.  Hiroaki T, Tomohiro I, et al. Ind Engng Chem Res 1995;34:1401±11.  Smoot LD. Fundamentals of coal combustion for clean and ef®cient use. Amsterdam: Elsevier, 1993.  Levin EM, Robbins CR, Mcmurdie HF. Phase diagrams for ceramists, vol. 1. The American Ceramic Society, INC., 65 Ceramic Drive, Columbus, Ohio 43214, 1964, 219.  Couch G. Understanding slagging and fouling in pf combustion. IEA Coal Res 1994;August.  Benson SA. Inorganic transformations and ash deposition during combustion, the American Society of Mechanical Engineering, 1992:209±28.  Steven ZD, Augus BU, et al. Fuel 1989;68:346±53.